When a bolt is screwed and clamped into a female screw by such as an impact wrench and the like, the clamping torque is generally determined so that when said clamping torque reaches to a prescribed value, the clamping of said bolt is set to be completed.
To determine said clamping torque, generally a method is employed, using a bolt having flat plane faces at the upper end of the bolt head and at the end of the bolt shaft, and making the supersonic wave sending and receiving probe of an axial force detector come into contact with the flat plane face of the upper end of said bolt head, to send the supersonic wave generated in said axial force detector into the bolt through said supersonic wave sending and receiving probe from the upper end of the bolt head, so as to reflect said supersonic wave onto said flat plane face at the end of the bolt shaft, then receiving said reflected supersonic wave with said supersonic wave sending and receiving probe to measure the time elapsed from the input of said supersonic wave to the receiving of the reflected supersonic wave, with the axial force of the bolt being calculated based on said elapsed time.
To measure said axial force correctly, the flatness and perpendicular degree of both said flat plane faces of the upper end of said bolt head and the end of said bolt shaft should be as accurate as possible, and the surface roughness of both said flat plane faces of the upper end of said bolt head and the end of said bolt shaft should be minimal.
On the other hand, a polygonal (generally hexagonal) shaped outline is given to the bolt head so that the bolt can be clamped effectively into the female screw with the bolt clamping machine, it being desirable that said polygonal shaped outline of said bolt head be angular enough.
Hitherto, a recess is formed on the bolt head to give an angular polygonal shaped outline to said bolt head.
More particularly, in general, to form the bolt head having polygonal shaped outline, cold forging of the columnar bolt material in a forging mold has been applied, and in said cold forging process, a bolt head forging mold having a projecting mold part to form the recess on the upper face of the bolt head is used, and when the recess on the upper face of the bolt head is formed by said bolt head forging mold's projecting mold part, the material of said bolt head flows radially giving an angular polygonal shaped outline to said bolt head. Further, the end part of the bolt shaft has a tapered shape, so as to reduce gradually the diameter of the end part toward the end of the bolt shaft so as to guide easily the bolt into the female screw. Said tapered end part of the bolt shaft is called the C-face part.
Patent Literature 1Patent Publication JP2007-301663Patent Literature 2Patent Publication JP2002-316235Patent Literature 3Patent Publication JP2002-113546
As described above, to give an angular polygonal shaped outline to said bolt, a recess is formed on the bolt head by the cold forging (see Patent Literature 1), a plane face being formed on the bottom of said recess to measure the axial force by a supersonic wave and the like, with the supersonic wave sending and receiving probe of said axial force detector being attached to said plane face of said recess.
Nevertheless, in a case where the diameter of said probe is larger than that of said recess, said probe can not be put into said recess so that the size of said probe is limited by the size of said recess.
Further, to form a wall around said plane end face of said bolt shaft, a die having a penetrating hole into which the shaft part of the bolt material is to be inserted is used, the diameter of the end part of said penetrating hole being set to gradually reduce so as to become a forming part of the outside of said wall (C-face forming part).
Using said die, the shaft part of the bolt material is inserted into the penetrating hole of said die, and punching is performed on said bolt material to form the tapered end part, the outer diameter of which reduces gradually toward the end of said shaft part of the bolt material. At the same time, a projecting end face is formed at the end of said tapered end part. When said projecting end face is pressed strongly with the correcting tool, said projecting end face is corrected to be a plane face (see Patent Literature 2).
In this traditional method, the multi-stage process consisting of forming said tapered end part (C-face part) by the cold forging, forming said plane projecting end face, and forming a spiral thread around said shaft part is necessary.
The method in which said tapered end part (C-face part) and said plane end face are formed simultaneously has been provided, said method using a die having a penetrating hole, the diameter of the end part of which is reduced gradually to be a C-face forming part, the bolt material being inserted into said penetrating hole, following which punching is performed on said bolt material to form the C-face part at the end of the shaft part of said bolt material, and at the same time, punching is further performed on the end of the shaft part of said bolt material to form a recess having a plane bottom face (see Patent Literature 3).
In said method, in which said tapered end part and said plane end face are formed at the same time, said plane end face has a small area, since said plane end face is formed in the center of the end of the shaft part of said bolt material, making it difficult to measure correctly axial force.